Method of bonding graphite to metal

Abstract: The present invention provides a method for bonding graphite (1) to metal (3) includes a step of brazing the graphite (1 ) to the metal (3) with an active alloy brazing material (2), characterized by the step of forming a layer (1a) on a surface of the graphite (1) before bonding the graphite (1) to the metal (3), the layer (1a) being composed of a carbide of a metal (3) to be bonded to the graphite (1).

Description: The invention relates to a method for bonding graphite to metal, and more particularly to such a method using an active alloy brazing process. The graphite or black lead has various superior properties, for instance, high thermal conductivity, low thermal expansion coefficient, low secondary electron emission coefficient and high emissivity. Thus, it has been researched so far to apply the graphite, for instance, to walls of a plasma container for a nuclear fusion reactor, a collector electrode of an electron tube, and capillaries of an ion laser tube. In these applications, a member made of graphite is supported by or secured to other members made of material other than graphite, in almost all cases, metal, by any means. When high mechanical strength and high thermal conductivity are required for such support and securing, a brazing process is usually chosen. As is understood from the fact that the graphite is suitable for walls of a metal fusion furnace and a crucible, the graphite has inherent property of not reacting on molten metal. However, the graphite reacts on an active metal such as Ti, Zr, Ta and Mo at a temperature over 1,000 degrees centigrade, and thus forms a layer composed of a carbide of these metals at an interface thereof, to thereby firmly bond with the metal. For instance, JP-A-62-21768 published on January 30, 1987 teaches that graphite can be bonded to Mo or Ni-Cr-Fe alloy including Mo by more than 8% in an inert gas atmosphere, for instance, under heat treatment at a temperature of 1,400 degrees centigrade for 90 hours. JP-A-63-310778 published on December 19, 1988 discloses a process in which a surface of a member composed of graphite is first metalized with active brazing alloy having Ti-Cu-Ag structure, and then brazed with brazing alloy such as Ag, Au and Ni, to thereby form a stress relaxation layer for bonding the graphite to metal. In the aforementioned brazing processes in which an active alloy brazing material is to be used, since the active alloy reacts on and thereby bonds to the graphite only within contact area of each other, it is necessary for bonding to apply to the contact area sufficient pressure, for instance, in the range of 200 grams per square centimeter or more and also temperature of approximately 1,000 degrees centigrade. In addition, since the reaction develops at relatively slow speed under the high temperature, it takes relatively long time for brazing. This increases probability of coarsening of crystal grains and/or deformation due to creep in metal parts or jigs. Accordingly, it was required to fabricate the metal parts or jigs tougher for preventing such crystal grain coarsening and deformation. As above mentioned, in a process utilizing solid phase reaction by active metal, the reaction develops first at the contact surface of the graphite and the active metal, and then the bonding area enlarges due to thermal deformation and diffusion. Accordingly, it is necessary to apply a pressure over approximately 100 grams per square centimeter and also high temperature to area along which the graphite is to be bonded to the metal, and further it is necessary to continue such brazing equipments for long hours. For instance, the aforementioned JP-A-62-21768 teaches that it needs high temperature and long time, specifically the temperature of 1,400 degrees centigrade for 90 hours, for accomplishing to bond the graphite with the metal. Such temperature is equal to or higher than the melting point, or higher than two third of the melting point of ordinary industrial metalic materials such as oxygen free copper, copper-nickel alloy, stainless steel and iron-nickel alloys. In addition, since the metallic materials are heated for long time (hours), melting of the metal, recrystallization of metal crystals and coarsening of crystal grains are remarkable, and thereby remarkably decrease the mechanical strength of the resultants. In the case that the above mentioned process is to be applied to parts for use of an electron tube, there arise problems that metal parts are deformed due to high temperature creep,and that brazing materials in particular silver and gold develop to an alloy to thereby cause erosion of the brazing materials. These cause inaccuracy of dimensions and higher probability of occurrence of inferior sealing. In the process disclosed in the above mentioned JP-A 63-310778, in which the surface of a graphite member is in advance metallized with Ti-Cu-Ag brazing material, it takes relatively short time for brazing since the surface is metalized. However, in the case that brazing materials are to be used for areas to be exposed to electron beams or high temperature gas plasma, such as a collector of an electronic tube and a gas laser tube, the metallized surface inversely deteriorates the secondary electron emission and plasma-resisting property to the degree worse than the graphite. EP-A-0115577 discloses a method acccording to the preamble of claim 1. According to this method the side of a graphite plate intended for a bonding is covered with a carbide former (i.e. a metal), a brazing solder is applied and the lamination is applied to the counter surface and heated to brazing temperature at least in the bonding zone and thereafter cooled. In an alternative method a brazing-suitable metal layer is applied by electroplating, vapour deposition or spraying instead of using a carbide former. Chemical abstracts, vol. 113, no. 22, 26 November 1990, Columbus, Ohio, US; abstract no. 196591n, "Brazing of pyrolytic graphite parts to metal parts", page 325, discloses the application of an alloy brazing material which includes Ti. It is an object of the invention to provide a method for bonding graphite to metal without degradation of mechanical properties, inaccuracy of dimensions and inferior sealing and also without degradation of secondary electron emission and plasma-resisting property. This object is achieved by a method defined in claim 1. Preferred embodiments of the method are claimed in claims 2 to 6. In a preferred embodiment, the metal carbide layer is composed of any one of metal carbides selected from the group consisting of TiC, TaC, ZrC and NbC. In another preferred embodiment, the metal carbide layer has thickness in the range of 25 micrometers to 300 micrometers. In still another preferred embodiment, the metal carbide layer is formed by chemical vapor reaction (CVR) process. In yet another preferred embodiment, the active brazing alloy includes Ti. In still yet another preferred embodiment, the graphite has thermal expansion coefficient which is different from that of the metal carbide layer by 1 × 10 -6 per degree or less. The inventor has conducted many experiments and analysis to solve the above mentioned problem, and then has found the fact that if a metal carbide layer is in advance formed on a surface of a graphite member, an active metal such as Ti could more easily react on the metal carbide layer to thereby bond the metal to the graphite, rather than the case in which a graphite member is made to directly react on an active metal such as Ti to thereby bond the graphite member to the metal. In other words, the present invention is based on the fact that the wettability or reactivity between an active metal such as Ti and metal carbide is greater by one order or more than the wettability or reactivity between an active metal and graphite. For instance, if a member composed of graphite is covered at its surface with TaC and NbC layers having thickness of 50 micrometers, Ti-Cu or Ti-Au active alloy brazing material can flow out to and spread in the surface by capillary phenomenon when one through two minutes have passed after the brazing material had melted, similarly to the case in which metal such as Cu and Mo are to be brazed to graphite. Thus, the graphite is bonded to the metal. Thus, the present invention provides a new method for reforming a surface layer of a graphite member, in which method a carbide layer is in advance formed on a surface of a graphite member. As explained later in detail, the method in accordance with the invention has many advantages. More specifically, the method can provide a graphite-metal bonding structure having high intensity, high heat-conductivity, high radiation rate and low secondary electron emission coefficient all of which cannot be obtained by a conventional metal surface layer or metalized layer. The method for bonding graphite to metal in accordance with the invention can be applied to a collector or an anode of an electron tube which receives electron bombardment, capillary or parts disposed between gas lasers and exposed to high plasma of more than 2,000 degrees centigrade, and inner walls of a nuclear fusion reactor. The advantages obtained by the aforementioned present invention will be described hereinbelow. In the present invention, a metal carbide layer is first formed on a surface of a graphite member. This much increases the wettability or reactivity and flowability of a brazing material relative to a conventional direct brazing. Thus, it is possible to braze by preplaced brazing similarly to the brazing metal to other metal. Accordingly, the brazing material is not limited to a board with respect to a shape thereof, and hence it is possible to use brazing materials having shape of wire, rod, powder and paste. Accordingly, it is easier to design parts for brazing, and in addition, it is no longer necessary to apply a pressure of approximately 300 grams per square centimeter or more to a surface along which graphite is bonded to metal. Hence, it can advantageously simplify jigs and parts for brazing. Furthermore, since the time period for brazing is in the order of several minutes, no consideration needs to be taken for thermal loss and mechanical degradation of jigs and parts for brazing. This enables to much shorten the time period for brazing, and hence enhances operation rate of a vacuum brazed equipment much times and further lengthens lifetime of the equipment. The metal carbide layer formed in advance on a surface of a graphite member has various advantages. For instance, if the present invention is applied to an electron tube for microwaves or parts for a gas laser tube, it is possible to prevent the occurrence of inferior sealing due to dusts and to depress secondary electron emission, and further to enhance plasma-resisting property. As is clear to those skilled in the art, it is obvious that the present invention can use any method for forming a metal carbide layer on a surface of a graphite member. For instance, a chemical vapor deposition (CVD) process and a physical vapor deposition (PVD) process such as sputtering and ion plating are suitable for that purpose. Fig. 1 is a cross-sectional view for explaining an embodiment in accordance with the invention in which a graphite member is to be bonded to a metal member. Fig. 2 is a microphotograph showing a structure of an area in which the graphite member is bonded to the metal member. A preferred embodiment in accordance with the invention will be explained hereinbelow with reference to drawings. Fig. 1 is a cross-sectional view of an area in which a graphite member 1 is to be bonded to a metal member 3 constituted of Mo. Fig. 2 is a picture showing the structure of the bonding area and taken by an optical microscope. As illustrated in Fig. 1, at first, the graphite member 1 is formed at its surface with a layer 1a constituted of TiC and having thickness of 50 micrometers by means of a chemical vapor reaction (CVR) process. Then, overlaid on the TiC layer 1a is a brazing plate 2 constituted of Ti by 3% and Au by 97% and having thickness of 0.1 millimeter, and further overlaid on the brazing plate 2 is a metal plate 3 composed of Mo and having thickness of 0.3 millimeters. Then, placed on the metal plate 3 is a weight 4 of approximately 150 grams, and subsequently, the entire resultant is placed in a vacuum brazing furnace and then was brazed. The brazing was conducted maintaining the degree of vacuum in the vacuum brazing furnace not more than 2 × 10 -4 Pa. The furnace was heated up to approximately 950°C within approximately one hour and a half. Observing the inside of the furnace through an observation window, the heating was stopped after it was confirmed that the brazing plate 2 started to melt and flow out. It was observed that the brazing plate 2 started to melt at the temperature of 1,030°C, and it took about 3 minutes for the brazing plate 2 to wholly melt and flow into between the metal plate 3 and the graphite part 1. This period of time was almost the same as that necessary for brazing metal to other metal, for instance, brazing Mo to oxygen free copper. Then, the bonded structure was cooled down to 300°C within about 40 minutes, and subsequently further cooled down to the room temperature by introduction of nitrogen gas. The brazing plate 2 was perfectly melted. As is understood from Fig. 2, a 200 times magnified picture taken with an optical microscope and showing the bonding structure, the wholly melted brazing plate 2 becomes a brazing material layer 2a and forms a wet or reactive fillet together with the metal part 3 and the graphite 1. Thus, it can be observed that the brazing material layer 2a is bonded at its entire top surface to the metal part 3 and is also bonded at its bottom surface to the TiC layer 1a and hence the graphite 1. Hereinbelow will be explained an analysis with respect to wettability or reactivity of a brazing material disposed on a metal carbide layer. As explained above, the inventor has found the fact that the wettability or reactivity between a metal carbide layer and an active metal brazing material is much better than that between a graphite and an active metal brazing material, and thus it is industrially preferable to add a step of forming a metal carbide layer. In accordance with this discovery, the inventor conducted an experiment with respect to other metal carbides which can be formed on a graphite. The results are shown in Table 1. Sample No. Metal Carbide Thickness (µm) Brazing Material Brazing Temp. (°C) Contact Angle (degree) 1 TiC 100 3%Ti-Cu 1055 12-34 2 TaC 50 3%Ti-Cu 1055 10-40 3 TaC 85 2.3%Ti-Cu 1060 7-38 4 ZrC 50 2.3%Ti-Au 1060 10-30 5 ZrC 70 3%Ti-Cu 1055 9-35 6 NbC 50 Ti-Cu-Ag 830 5-30 7 No layer (for reference) 2.3%Ti-Au 1060 55-120 8 No layer (for reference) 3%Ti-Cu 1055 50-135 It has been found based on the experimental results shown in Table 1 that the brazing materials including Ti has the superior wettability or reactivity to various metal carbides. It should be noted that the thickness of the metal carbide layer much influences the adhesion between the metal carbide layer and the graphite disposed below the carbide layer. In particular, a smaller difference in thermal expansion coefficient between the graphite and the metal carbide layer causes less possibility of peeling and cracking. Accordingly, if the difference is sufficiently small, the metal carbide layer can have greater thickness. According to the experiments conducted by the inventor, the difference in thermal expansion coefficient between the graphite and the metal carbide layer is preferable 1.5 × 10 -6 per degree or less, and more preferable 1 × 10 -6 per degree or less. The thickness of the metal carbide layer has to be determined taking into consideration the structure of a brazed equipment and conditions for use thereof. If the thickness is equal to or less than 25 micrometers, the metal carbide layer may not entirely cover irregularities of a surface of the graphite 1, and if the thickness is over 300 micrometers, the adhesion between the graphite and the metal carbide layer is deteriorated. Thus, it is preferable to arrange the thickness of the carbide layer in the range of 25 to 300 micrometers.

Claims: A method for bonding graphite (1) to metal (3) including a step of forming a layer (1a) on the surface of the graphite (1) before brazing the graphite (1) to the metal (3) with an alloy brazing material (2), characterized in that said layer (1a) is composed of a metal carbide and has a thickness of at least 25µm. A method in accordance with claim 1, wherein said metal carbide layer (1a) is composed of any of metal carbides selected from the group consisting of TiC, TaC, ZrC and NbC. A method in accordance with claim 1 or 2, wherein said metal carbide layer (1a) has thickness in the range of 25 micrometers to 300 micrometers. A method in accordance with any of the preceding claims, wherein said metal carbide layer (1a) is formed by chemical vapor reaction process (CVR), or a chemical or physical vapor deposition process (CVD, PVD). A method in accordance with any of the preceding claims, wherein said alloy brazing material (2) includes Ti. A method in accordance with any of the preceding claims, wherein said metal carbide has a thermal expansion coefficient which is different from that of said graphite (1) by 1 x 10 -6 per degree or less.

Inventors: KASAHARA AKIHIKO, TASHIRO MINORU, KASAHARA, AKIHIKO, TASHIRO, MINORU, KASAHARA, AKIHIKO, C/O NEC CORPORATION, TASHIRO, MINORU, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1994
Main classification: C04B37
IPC codes: C04B41, C04B37, B23K35, B23K1